Duchenne Muscular Dystrophy (DMD)


Becker muscular dystrophy (BMD) is an X-linked genetic disease characterized by progressive muscle weakness and atrophy. It is 1 of 9 types of muscular dystrophies affecting between 1 in 18,000 and 1 in 30,000 male births.1

Becker Muscular Dystrophy History

BMD was first described by the German physician Peter Emil Becker in the 1950s as a variant of Duchenne muscular dystrophy (DMD).2 

Becker Muscular Dystrophy Cause

BMD is caused by mutations in the DMD gene.3 However, unlike in the case of DMD, which is severe and where cells can produce no functional dystrophin protein because of mutations in the DMD gene, in BMD, the dystrophin protein conserves some of its function. This is because the deletions or other mutations do not usually disrupt the reading frame of the gene and cells can still produce a shorter but still functional dystrophin protein.4

The dystrophin protein plays a key role in stabilizing and protecting muscle fibers during muscle contraction.5 So, when it does not function properly, muscle cells become damaged and atrophy sets in over time. This leads to muscle weakness and wasting, as well as cardiomyopathy.

Becker Muscular Dystrophy Inheritance

Like DMD, BMD is also inherited in an X-linked manner.6 The DMD gene resides on chromosome Xp21.2. 

Females who have 1 copy of the mutated DMD gene are called carriers. They may have some symptoms of the disease but these are usually mild. They have a 50% chance of passing the mutated gene onto their children. So they have a 25% chance of having a son affected by the disease and a 25% chance of having a daughter who is a carrier. They also have a 50% chance of having children who do not inherit the faulty gene.

A male with BMD will not pass the faulty gene to any of his sons, as males only receive Y chromosomes from their father, but will to all of his daughters, who will be carriers of the disease. 

Becker Muscular Dystrophy Symptoms

The symptoms of BMD are similar to those of DMD but are usually milder and appear later, usually when patients are in their teens though they can appear any time between ages, 5 and 60.7 Muscle weakness usually begins in the hips and pelvis and then progresses to the thighs and shoulders. Contracture and muscle cramps are also common. As the disease progresses, patients may eventually need a wheelchair for mobility. 

The disease can also affect the heart muscle. The most common cause of death is dilated cardiomyopathy, which usually occurs when patients are in their 40s.8 

Becker Muscular Dystrophy Diagnosis

Like with other forms of muscular dystrophy, the diagnosis of BMD begins with a physical examination and a family history of the patient.7 Electromyogram and nerve conduction studies can be performed to understand whether muscle weakness is caused by a problem in the muscles themselves or motor neurons. 

Levels of creatine kinase (CK) in the serum can indicate muscle damage. Although in male patients with BMD, CK levels are usually up to 5 times the upper limit of normal levels, this alone cannot differentiate BMD from other muscular dystrophies. In female carriers of BMD, CK levels vary between 2 and 10 times the normal concentration.9

A muscle biopsy can show signs of muscle degeneration and hypertrophy but this is not specific to BMD. Western blot analysis of the biopsy sample can reveal the amount of dystrophin protein, and help estimate the severity of the disease. Patients with severe BMD have between 5% and 20% of normal dystrophin levels, and those with mild to moderate BMD have between 20% and 50%, and 20% and 100% of normal dystrophin levels, respectively.9 

Genetic testing analyzing the DMD gene can be used to reach a final diagnosis. The DMD gene can be analyzed in terms of large deletions or duplications (the most common genetic cause of the disease, seen 70% to 80% of cases) or sequenced to look for more subtle changes.7

Becker Muscular Dystrophy Treatment

There is currently no cure for BMD but corticosteroids can be prescribed to delay the progression of the disease and help patients maintain their ambulation for as long as possible.10 However, corticosteroid use can cause serious side effects, especially if used for a long time. 

Physical and occupational therapy can help patients maintain muscle strength for as long as possible and minimize contracture. Some patients with contracture or scoliosis may need surgery. 

Patients’ heart function should also be monitored closely using electrocardiograms and echocardiograms. In severe cases, cardiomyopathy (disease of the heart muscles) can lead to heart failure and patients may need a heart transplant. 

References

  1. Becker muscular dystrophy. Genetic and Rare Diseases Information Center. Accessed July 8, 2021.
  2. Becker Muscular Dystrophy (BMD). Muscular Dystrophy Association. Accessed July 8, 2021.
  3. DMD gene. MedlinePlus. Accessed July 8, 2021.
  4. Types of mutations. Parent Project Muscular Dystrophy. Accessed July 8, 2021.
  5. Gao Q, McNally EM. The dystrophin complex: structure, function and implications for therapy. Compr Physiol. 2015;1;5(3):1223–1239. doi:10.1002/cphy.c140048
  6. Causes/inheritance. Muscular Dystrophy Association. Accessed July 8, 2021.
  7. Diagnosis. Muscular Dystrophy Association. Accessed July 8, 2021.
  8. Muscular Dystrophy, Becker. National Organization of Rare Disorders. Accessed July 8, 2021.
  9. Darras BT, Program N, Miller DT, et al. Dystrophinopathies. In: Adam MP, Ardinger HH, Pagon RA, et al, eds. GeneReviews®. University of Washington, Seattle; 2000. Updated April 26, 2018. Accessed July 8, 2021.
  10. Becker Muscular Dystrophy. Johns Hopkins Medicine. Accessed July 8, 2021.

Reviewed by Debjyoti Talukdar, MD, on 7/1/2021.

READ MORE ON DMD